The European eel has been in decline since the early 1980s. Fewer silver eels returning to sea to spawn, as a result of reduced spawner production and human activities along the reproductive migratory route, is often considered to be a significant contributing factor to the decline. Consequently, the EU has introduced a conservation target of 40% escapement of silver eels from each European river system. Escapement rates of migrating silver eels must therefore be quantified in each EU member state’s eel management units to aid recovery to such a state. Downstream migration of European silver eels (Anguilla anguilla, L.) leaving Lough Neagh, Northern Ireland, was investigated using acoustic telemetry in the out-flowing lower River Bann. The acoustic telemetry was intended to corroborate the use of a traditional mark/recapture method using numbered identification tags for estimating silver eel escapement from the system. Similar recapture rates between both tagging methods verified that the traditional method is comparable to hydroacoustics for assessing fisheries recapture rates of silver eel, from which escapement estimates are derived. 40% of acoustically tagged eels were not detected (i.e. did not migrate), with evidence suggesting that some of these could have migrated outside of the study period, or after transmitter battery failure. 53% of eels that did migrate downstream and escaped past the fisheries did not reach the sea, suggesting that the traditional method may be overestimating escapement. However, it is likely that some eels ceased migration and eventually proceeded towards the sea after this study had ended. Longer-term studies using additional tracking equipment are needed to investigate the behaviour of non-detected eels and reveal whether those not recaptured by fisheries that do not reach the sea are delaying migration or are suffering mortality.